Separation of solid materials in mechanical flotation machines is effected by agitation of a pulp containing a material to be floated using an impeller installed in a flotation cell into which the pulp is fed. Agitation aerates the pulp thus dispersing the air contained in the pulp with the result that air bubbles develop to which particles of the material being separated (floated) stick. These particles rise, together with the air bubbles, to the pulp surface to form a froth product having a higher concentration of the floatable material as compared to the starting product. The froth product is fed through an overflow lip of the flotation cell to further treatment.
A prior art flotation machine (US, A, 3393802) comprises a flotation cell adapted for receiving a pulp containing a material to be floated and having in its upper portion an overflow lip to carry off the froth product therethrough, a circulating pipe vertically installed in the cell and having its lower end disposed in the lower portion of the cell, an impeller of the centrifugal type installed between the lower end of the circulating pipe and the cell bottom so as to permit rotation about the vertical axis, with the impeller inlet side facing the lower end of the circulating pipe, for the pulp to move from the interior of the circulating pipe to the space at the outlet side of the impeller, openings disposed in the wall of the circulating pipe in the upper portion of the flotation cell and communicating the interior of the circulating pipe with its environment in the cell and thus with the space at the outlet side of the impeller, and an impeller drive means connected to the impeller through a hollow shaft. The centrifugal impeller is constituted by a horizontally disposed disc with blades in radial arrangement on its top. The impeller is enclosed by a stator constituted by a fixed horizontal ring attached to the lower end of the circulating pipe and by vanes radially arranged around the impeller near the edges of its blades and attached to the outer lateral surface of the ring.
As the impeller rotates, the pulp from the interior of the circulating pipe is passed into the region between the radial blades of the impeller and, by the centrifugal action, discharged from the impeller outlet side through the annular gap between the impeller disc and stator into the lower portion of the flotation cell. The pulp is passed through the openings in the walls of the circulating pipe in the upper portion of the cell back into the circulating pipe. Air is supplied under pressure to the pulp from an external source through the hollow shaft of the impeller. Air dispersion occurs as the pulp and air flow round both impeller blades and stator vanes.
With this design of a flotation machine, pulp is agitated with approximately equal intensity throughout the whole volume of the flotation cell, i.e. in the bottom portion of the flotation cell (agitation zone) as well as in its remaining portion (flotation zone). Intense agitation of the pulp in the flotation zone renders the solid particles of the material being floated more apt to tear off from air bubbles, which impairs the outlet of the flotation machine; on the other hand, intense agitation of the pulp in the flotation zone causes materials contained in the pulp that are not intended to be floated, such as gangue, to go to the pulp surface, which impairs the quality of the concentrate obtained, i.e. lowers the content of the material separated.
Agitation of the pulp in the flotation zone in such a machine requires additional power. It is also known that impellers of the centrifugal type are of relatively low efficiency.
The rotational speed of a centrifugal impeller is limited by cavitation. Also, increase in the rotational speed of the impeller results in a cone of influence rotating in the same direction as the impeller, which breaks the flow and brings to an end the agitation of pulp in the agitation zone. This necessitates the use of a reduction gear to transmit the rotation of a motor with relatively high rotational speed to the shaft of a centrifugal impeller with relatively low rotational speed, which increases the metal input per structure. Decrease in the rotational speed of the impeller also decreases the number of air bubbles formed by air dispersion at the impeller outlet, which is caused by a decreased number of collisions between pulp and impeller blades or stator vanes, thus lowering the machine efficiency.
The use of an external source for forces aeration of pulp involves additional equipment and increases power consumption.
Also known in the prior art is a flotation machine (N.F. Mescheriakov, "Flotatsionnye Mashiny i Apparaty," 1982, Nedra (Moscow), pp. 97-103) comprising a flotation cell adapted for receiving a pulp containing a matter to be floated and having in its upper portion an overflow lip to carry off the froth product therethrough, a circulating pipe mounted in the cell and passing from top to bottom so that its lower end is disposed in the lower portion of the flotation cell, an impeller of the centrifugal type rotatably mounted above the vertical axis between the lower end of the circulating pipe and the bottom of the flotation cell, with its inlet side facing the lower end of the circulating pipe, to permit pulp movement from the interior of the circulating pipe to the space at the outlet side of the impeller, and an impeller drive means. In this machine, the space at the outlet side of the impeller communicates with the interior of the circulating pipe through its open end disposed in the upper portion of the flotation cell below the overflow lip. Horizontally mounted within the flotation cell is a grid separating the upper portion from the lower portion of the cell. Air supply is effected by suction from atmosphere as a result of rarefaction in the circulating pipe due to rotation of the impeller, which eliminates the need of a separate source for compressed air supply. The centrifugal impeller is similar in design to that of the aforementioned U.S. Pat.
In operation of such an apparatus, a fluidized bed of solid particles contained in the pulp is formed on the surface of the grid at the lower side thereof as a result of the impeller's feeding continuously aerated pulp flow, said particles being captured by air bubbles in the aerated pulp and going up.
The grid makes it possible to separate the agitation zone and flotation zone of the flotation cell, thus decreasing the agitation intensity in the agitation zone. This makes it possible, on the one hand, to lower the probability for the solid particles of a material being separated to be torn off from air bubbles in the flotation zone, thus raising the machine efficiency, and, on the other hand, to decrease the amount of materials which are not intended to be floated but rise to the pulp surface, thus improving the quality of the concentrate obtained as compared to the apparatus of the afore-mentioned U.S. Pat.
However, sufficiently intense agitation of pulp particles on the grid surface is required to attain reasonable output, which results in considerable turbulence of pulp flows maintained in the flotation zone and impairs the concentrate quality and machine output and increases power consumption as indicated above.
The use of a centrifugal impeller results in high consumption of power and metal due to the use of a reduction gear and impairs air dispersion. Also, in such a flotation machine, additional power is required to overcome the resistance offered by the grid to the flow of pulp passing therethrough as well as to create above the impeller a rarefaction zone for air suction from atmosphere.
Moreover, the metal grid of considerable size and weight further increases the metal input of the machine.